This invention relates to superplastic forming of sheet metal using a self supporting ceramic superplastic forming die, and more particularly to a ceramic forming die which provides for catastrophic decompression control, peripheral system integration, leak prevention where die penetration is desired, and non-coplanar contact surface geometry. Additionally, this invention relates to damage tolerant contact surfaces for ceramic dies, and to superplastic forming processes using ceramic dies to provide various advantages such as part cavitation prevention.
Superplastic forming is well known and is used throughout the aerospace industry as well as in other industries to form sheets of titanium, steel, and aluminum. Prior to the superplastic forming process, these forming operations were often performed using lead hammer forming. This process uses a lead punch or hammer to drive the material to be formed, the "workpiece," into a forming die. The punch and die are not only expensive to make, but also environmentally undesirable both because the process is extremely noisy, and because it created airborne heavy metal and lead dust. The advent of superplastic forming has allowed a great many parts formerly produced using lead dies to be produced using less environmentally adverse die materials in a far quieter process. Thus, facilitating the transition from archaic hammer forming techniques to superplastic forming would be extremely useful for the industry.
Superplasticity is a metal's capability at certain temperatures and strain rates to exhibit very high elongation rates while avoiding localized thinning. At the limits of traditional forming processes the work piece ceases to elongate uniformly and begins to deform in discreet places. This tendency is generally referred to as "necking" and is undesirable because a work piece which has necked down in a specific location will be more prone to fail prematurely at that location when put under load. A superplastically formed part may both avoid localized necking and undergo far greater elongation than otherwise possible. This increased elongation makes forming more complex parts possible. It also makes possible a reduction of part count by integrating multiple parts, which conventionally would be riveted into one assembly, into a single superplastically formed part.
The superplastic forming process may be combined with diffusion bonding, laser welding, or resistance welding to produce complex sandwich structures under superplastic conditions. Diffusion bonding refers to the process of laminating two or more sheets of superplastically formable material together with the bonds typically only occurring in a discrete pattern such as a lattice. During the forming process, gas pressure is applied between the sheets to push them apart where they are not bonded. The resulting part, a truss core sandwich, consists of two or more sheets supported internally by diagonal braces. This process creates parts with design features never achieved prior to the combination of superplastic forming and diffusion bonding. Laser and resistance welding are substantially similar to diffusion bonding in that, before forming, multiple sheets of material are welded together at discrete locations using the laser welding process rather than diffusion bonding. After welding, a truss core sandwich can be produced using superplastic forming.
Superplastic forming dies are typically made of corrosion resistant steel (CRES) in order to withstand the high temperature and pressure associated with superplastic forming. While CRES is very durable and has been a useful material for superplastic forming dies, machining CRES dies is very time consuming and expensive. A great deal of effort has gone into finding replacement material for CRES in superplastic forming dies, directed primarily toward the use of ceramics in superplastic forming dies. Prior efforts have included a wide range of improvements from simply using a ceramic male insert in a CRES die to using a CRES containment vessel with the entire formed shape made from a ceramic insert.
Ceramic forming dies have been a great asset in developing die configurations. It is possible to avoid committing the resources necessary to make a CRES production superplastic forming die until the die geometry has been fully developed using ceramic dies in an external pressure vessel. The ideal superplastic prototype forming die would wholly eliminate the use of CRES and avoid the associated machining costs, material waste, and part size limitations created by pressure vessels.
Among the reasons for pursuing the use of free standing ceramic forming dies is both that ceramic is far less expensive to fabricate than CRES, and that, unlike CRES, ceramic die forming and disposal pose little environmental impact. However, prior art ceramic dies necessitated a pressure vessel to prevent the die from bursting when subjected to superplastic forming pressure. See e.g. Caldwell, U.S. Pat. No. 5,016,805. A containing pressure vessel would have to be machined from CRES and then either inserted into a hydraulic forming press, or fitted with a complex securing method to insure proper support of the internal ceramic forming die. See e.g. Leonard, U.S. Pat. No. 4,584,860. Dedicating die space to the pressure vessel limits the maximum part size. Furthermore, pressure vessels restrict the die periphery to a certain shape which defines the initial work piece size and may consequently result in considerable material waste. A superior die arrangement would allow the die to take whatever external shape was best suited to the particular part to formed.
External pressure vessel use protects die operators from injury caused by potentially explosive decompression in the event of failure of the ceramic die. The forming die may experience a dramatic pressure spike if the work piece ruptures or tears out while being formed, especially if high differential pressure is being applied to form the work piece. In such event, a sudden increase of pressure will occur in the die, subjecting it to substantial impact stress. The pressure vessel was perceived to be necessary in part because of the potential for uncontrolled catastrophic die failure and because of the concomitant inability to insure controlled release of superplastic magnitude pressures that could result from pressure spikes during the superplastic forming process. This unpredictable die failure potential was believed to make use of self supporting ceramic dies undesirably hazardous. A preferable solution would eliminate the hazards of ceramic die failure but avoid resorting to the costly and cumbersome pressure vessel solution previously employed.
One factor which has delayed development of a self supporting ceramic superplastic forming die has been the inability to produce a die strong enough to avoid using an external supporting pressure vessel to carry the pressures involved in the forming process. For example, the die must withstand considerable compression force from the press. The press must apply sufficient force to secure the work piece periphery during forming and to seal the die and lid during forming to substantially prevent the escape of gas from the forming cavity. Several companies have devoted considerable time and money in hopes of developing ceramics and methods for making a ceramic die with sufficient strength and durability to survive the superplastic forming process. Unfortunately, no one has been able to achieve breakthroughs that would allow a ceramic superplastic forming die to be used without some sort of pressure vessel. This lack of useful development results principally from ceramic's particular susceptibility to fracture. Prior art ceramic dies are prone to this weakness partly because a large number of minor internal defects in the ceramic result from the prior art die manufacturing method. It would be desirable to develop a method for using existing ceramic material to make a superplastic forming die, yet avoid the necessity of placing that die in a pressure vessel.
A ceramic die's useful life has typically been limited to production of only a few parts; usually on the order of five or fewer, because of rapid die wear. For example, superplastically formed titanium which directly contacts the ceramic die seal surface tends to bond to that surface. When the formed titanium is subsequently removed from the die, a portion of the ceramic material that is bonded to the part is removed with the part. There is no prior art method for extending the die's seal surface life other than machining away a portion of the seal surface to make it sufficiently smooth to again form a proper seal. Ideally, ceramic dies would allow a longer production life by providing a way to protect the contact surface.
The contact surface of prior art superplastic forming dies is coplanar to simplify die sealing and fabrication. There have been some attempts to manufacture CRES dies or pressure vessels with contoured contact surfaces; however, only rarely was it worth the high machining costs to grind dies with contorted contact surfaces with sufficient accuracy that the two non-coplanar contact surfaces achieve a good seal surface. Exacerbating the problem, die creep and thermal distortion create sealing problems in non-coplanar dies after only a few part pressings. This limitation prevented both using a work piece that had some simple forming operation previously performed and using the dies themselves to non-superplastically form the work piece prior to the actual superplastic forming process. This resulted in two equally unsatisfactory alternatives. First, many potential part geometries could not be produced. The work piece contours that would be necessary to both produce the desired part and maintain the work piece periphery in the flat seal surface exceeded the limits of the superplastic process. Second, when production of such parts was attempted, the part would undergo excess thinning or wrinkling and be defective. It would be desirable to design a system with non-coplanar die contact surfaces without creating either high machining costs, or very short die life.
The conduits which do penetrate a ceramic die sometimes allow forming pressure to leak from the forming cavity by passing between outside of the penetrating conduit and the die hole. Various methods have been used to limit this such as swaging the conduit; however, maintaining a pressure tight seal at die penetration points has tended to require an undesirable high labor costs. A preferable technique would provide a simple method for preventing unintended die venting paths while increasing the reliability of such a system.
The current system of using a pressure vessel for ceramic dies is reliable and available, but it is expensive, requires high die maintenance costs, and tends to result in high die storage requirements. While it is conceptually possible to make an interchangeable pressure vessel work with many different ceramic dies, each die would have to be exactingly manufactured to insure proper alignment of pressure conduits, vent holes, quench conduits, power hook-ups, heating conduits, cooling conduits, and thermocouple holes or use of such devices would have to be eliminated. As a result, a specific pressure vessel typically must be dedicated to each die which substantially increases die cost because each die would require its own relatively expensive CRES pressure vessel. A self supporting die that could be inexpensively made for use on short production runs and discarded would substantially reduce die storage requirements. An improved die system that does not require the expensive pressure vessels and storage requirements would be of great benefit to the industry.
While use of ceramic in superplastic forming dies has advanced the art, the constraint of having to place ceramic in a CRES pressure vessel has hampered the rate at which the art could be advanced by making die fabrication more costly and difficult than a self supporting ceramic die would be. The need to use a pressure vessel results in part from fear that superplastic forming pressures could cause a self supporting die to explode unpredictably and cause harm of an unknown degree to both equipment and people. The value of ceramic dies to the industry would also be enhanced if there was a way to extend die life which is shortened by die to part bonding which quickly erodes the die. Superplastic forming use could also be expanded if the die contact surfaces could be shaped to conform more closely to finish part shape rather than be limited to flat contact surfaces. It would also be useful if the pressure differential between die cavities could be more closely controlled to prevent internal work piece cavitation. A superplastic forming die's value would also be enhanced by developing a simple way to not only integrate attachments, fittings, and lines directly into the die, but also prevent lines which penetrate the die from becoming die pressure loss paths.